1. Field of the Invention
The present invention relates to very pure polyisocyanates containing an isocyanurate moiety, and to the preparation of such isocyanurate/polyisocyanates by catalytic cyclotrimerization of aliphatic, alicyclic or arylaliphatic isocyanates.
More especially, the present invention relates to polyisocyanates containing an isocyanurate moiety which are prepared by catalytic cyclotrimerization of aliphatic, alicyclic or arylaliphatic isocyanates and which contain less than 0.03% by weight of residual isocyanate monomer and less than 1.0% by weight of dimer, based on the weight of the isocyanurate/polyisocyanate.
2. Description of the Prior Art
Many processes are known to this art for preparing polyisocyanates containing an isocyanuric group, by cyclotrimerization of isocyanates, in particular aliphatic, alicyclic or arylaliphatic diisocyanates.
These may be prepared, for example, by partial cyclotrimerization of the NCO groups of simple polyisocyanates or of polyisocyanate adducts, employing a variety of catalysts, such as tertiary amines (German Patent No. 951,168), derivatives of the alkali metals or alkaline earth metals, such as the hydroxides, carbonates or alcoholates (French Patent No. 1,190,065), quaternary ammonium hydroxides (French Patents Nos. 1,204,697 and 1,566,256; published European Patent Applications Nos. 0 3765 and 10 589), phosphines (French Patents Nos. 1,510,342 and 2,023,423), catalysts containing an ethylenimine group (French Patents Nos. 1,401,513 and 2,230,642) and Mannich bases (French Patents Nos. 2,290,459 and 2,332,274).
It is also possible to conduct this partial cyclotrimerization of NCO groups in the presence of aminosilyl compounds, such as the monoaminosilanes, diaminosilanes, silylureas and silazanes described in published European Patent Application No. 57,653.
The catalyst for the partial cyclotrimerization of NCO groups must, in general, be deactivated when the desired content of free isocyanate groups has been reached. Such deactivation may be accomplished by adding to the reaction medium an acidic compound (hydracid, acid chloride, etc.), an alkylating agent (methyl iodide, for example) or an acylating agent. This may also be accomplished by means of a suitable heat treatment.
Upon completion of the cyclotrimerization reaction, the polyisocyanate containing an isocyanurate moiety is separated by distilling off the unreacted isocyanate and, where appropriate, the solvent, when the reaction is conducted in a solvent medium.
For this purpose, it is necessary to heat the final reaction mixture, for example in an evaporator, and this permits removal of most of the residual isocyanate and, where appropriate, of the solvent. More often than not, it is then necessary to carry out an additional, more elaborate distillation in an effort to eliminate the final traces of isocyanate.
However, even after these various treatments, it is found that an amount of isocyanate at least equal to 0.1 or 0.2% by weight, based on the polyisocyanate, always remains in the final isocyanurate/polyisocyanate. In general, the polyisocyanate trimer also contains an amount of dimer equal to approximately 2 to 5% by weight based on the polyisocyanate.
In published European Patent Application No. 105,242, it has been proposed to reduce the content of residual isocyanate monomer by treating the polyisocyanate using 2 to 30% of an inert solvent, in a thin layer evaporator. This process nevertheless comprises a phase of heating the polyisocyanate to about 140.degree. C. to 150.degree. C., and entails the addition of a solvent which will then have to be removed.